1. Technical Field
The present disclosure relates generally to a fuel cell system and, more particularly, to apparatus and method for controlling a fuel cell system including a plurality of fuel cells.
2. Description of the Related Art
In a solid oxide fuel cell (SOFC) system used for generating medium and large capacity power of several ten kilowatts (KW) to several megawatts (MW), a plurality of small capacity fuel cell stacks are connected to each other so as to form the desired medium and large capacity. The fuel cell stack has a structure in which several ten to several hundred unit fuel cells are stacked and connected together so as to form desired output power.
When forming a fuel cell system by electrically connecting the fuel cell stacks to each other in series only, the fuel cell system is problematic in that, when even one stack is shut down because a problem occurs in the stack, the electric connection of all the fuel cell stacks of the fuel cell system is disconnected, so the fuel cell system is shut down. Accordingly, to avoid the above-mentioned problem, it is typical to form a fuel cell system by electrically connecting the stacks to each other in series and in parallel.
However, when a part of connected stacks in a fuel cell system in which the stacks are connected together in parallel is degraded, the degradation of the stack may be propagated to the other normal stacks, so the reduction in the durability of the fuel cell system is accelerated and the capacity of the fuel cell system is reduced, thereby reducing the life span of the fuel cell system.
FIG. 1A is a view illustrating propagation of degradation between fuel cell stacks when the stacks are connected to each other in parallel. FIG. 1B is a graph showing stack voltage as a function of stack current, in which the propagation of degradation between the fuel cell stacks when the stacks are connected to each other in parallel is shown.
As shown in FIGS. 1A and 1B, when one fuel cell stack is degraded in a fuel cell system in which a plurality of fuel cell stacks are electrically connected to each other in parallel (one stack degraded, see [2] of FIG. 2A), the internal resistance of the degraded stack is increased, so a leaning of current to normal stacks is generated. In other words, the flow of current in the degraded stack of the stacks connected to each other in parallel is restricted due to an increase in the internal resistance of the degraded stack, so the amount of current flowing in the degraded stack is reduced, but the amount of current flowing in normal stacks is increased compared to normal operation. Described in brief, when a stack of the fuel cell system is degraded, the current leans to the normal stacks, so the flow of current in the stacks becomes unbalanced. In this case, although the amount of current flowing in the degraded stack is reduced, the current leans to the normal stacks and the degradation is propagated to the normal stacks (primary propagation of degradation, see [3] of FIG. 2A).
In this case, the exothermic reaction of the degraded stack is reduced, resulting in a reduction in the temperature of the degraded stack, so the internal resistance of the degraded stack is further increased and the leaning of current to the normal stacks becomes worse, thereby increasing the propagation of degradation to the normal stacks and accelerating the degradation of both the degraded stack and the normal stacks (secondary propagation of degradation, see [4] of FIG. 2A).
FIG. 1C is a view illustrating propagation of degradation of a stack to normal stacks in a fuel cell system when stack degradation is accelerated. FIG. 1D is a graph showing the comparison of the life span of an abnormal fuel cell system, in which the stack degradation is accelerated, to the life span of a normal fuel cell system.
As shown in FIGS. 1C and 1D, when a part of stacks connected to each other in the fuel cell system is degraded and the degradation is accelerated, the degradation of the degraded stack is propagated to normal stacks, thereby reducing the durability, capacity and like span of the fuel cell system.
As the easiest method of avoiding the propagation of degradation of the degraded stack to the normal stacks, the currents and voltages of all the stacks in the fuel cell system may be independently controlled. However, to control the currents and voltages of all the stacks in the fuel cell system independently, the construction of the fuel cell system becomes complicated and the production cost of the system is increased. Particularly, to produce an SOFC system for generating medium and large capacity power, a plurality of stacks should be used, so it is problematic in that practical production of the SOFC system for commercial use, in which the stacks are equipped with respective power conditioning systems (PCS), is almost impossible.
Patent document 1 shown in the following “Documents of Related Art” discloses “a fuel cell system provided with a bypass circuit and a method of operating the system”. As shown in FIG. 2 of patent document 1, the invention of patent document 1 includes a bypass circuit 300 and a switching circuit 250 to 254, and is configured such that, when a degraded stack exists in the system, the bypass circuit bypasses the degraded stack using the switching circuit, thereby preventing the degraded stack from ill-affecting normal stacks.
However, the invention of patent document 1 discloses only the technique of bypassing a degraded stack, but does not disclose of a technique of reusing a degraded stack by rearranging the degraded stack.
Accordingly, it is required in the related art to propose apparatus and method for controlling a fuel cell system, which can easily and quickly connect fuel cell stacks to each other in series, in parallel or in series-parallel using cheap electric switches instead of using a plurality of power conditioning systems (PCS) during an operation of the fuel cell system, thereby preventing propagation of performance reduction between the stacks, improving durability of the fuel cell system, preventing a reduction in the capacity of the fuel cell system, and minimizing a reduction in the life span of the fuel cell system, and which can reuse degraded stacks by rearranging the degraded stacks with normal stacks, thereby efficiently using the fuel cell stacks.